Arrayed waveguide gratings (AWGs) are planar lightwave circuit (PLC) devices used for demultiplexing optical signals into individual wavelength channels. An AWG includes two slab-type star couplers, coupled to each other back-to-back via an array of planar waveguides of gradually increasing length. The gradually increasing waveguide length creates a gradually increasing optical delay on an inner surface of the output star coupler, which causes light at different wavelengths to couple into different output waveguides of the output star coupler. An AWG is a reciprocal device, that is, when used in a reverse direction, it can also combine wavelength channels into a common multiplexed signal. Thus, AWGs can be used for both multiplexing and demultiplexing of optical wavelength channels. Due to compactness and scalability of manufacturing, AWGs have found a wide application in optical networks.
One well-known drawback of AWGs is their thermal sensitivity. In an AWG, the optical path difference is created in the waveguide array, the refractive index of which depends on temperature. Because of this, center wavelengths of individual wavelength channels at the output of a silicon-based AWG drift with temperature, unless this drift is mitigated by some external means.
A common method to reduce AWG thermal drift is to stabilize the temperature of the PLC chip in which the AWG is formed. A heater and a temperature sensor are attached to the PLC chip. The temperature sensor is used to sense the PLC temperature. A temperature controller provides a signal to the heater to keep the temperature of the PLC chip constant. The temperature of the PLC chip is usually selected to be at the top of the required temperature range of the AWG device.
Temperature stabilization of AWGs has several drawbacks. One drawback is high electrical power consumption. Heaters having a power rating of at least several Watts are usually required to uniformly heat an AWG PLC chip. Another drawback is related to integration of thermally stabilized AWGs into a larger optical system. Heat released by the AWG heaters increases the overall system heat dissipation requirement, which calls for providing additional cooling means for the system. Furthermore, a time constant required for temperature stabilization and temperature tuning of heated AWGs is relatively large, typically ranging from few tens of seconds to few minutes.
Dragone in U.S. Pat. No. 5,920,663 discloses a method to reduce thermal drift of wavelength of an AWG by controllably deforming the PLC chip. The deformation stretches or compresses the optical lengths of the arrayed waveguides. Such changes give rise to birefringence effects that produce different propagation constants for the TE and TM waveguide modes. The deformation also provides some tuning of the transmission characteristics of the AWG, to correct for manufacturing tolerances. However, stress-induced birefringence increases polarization-dependent loss and polarization mode dispersion.
It has been recognized that an AWG can be tuned in wavelength by translating the input waveguide relative to the input star coupler of the AWG. Samiec et al. in U.S. Pat. No. 6,865,323 disclose an AWG device, in which an input waveguide is mounted on an expansion arm fixed on one end to a frame and having a holder on the other end. The expansion arm has a coefficient of thermal expansion (CTE) different from that of the frame. To restrict a movement of the input waveguide out of the PLC plane, a pair of flexible arms connect the holder to the frame. Detrimentally, the movable input waveguide in the Samiec device can cause the optical throughput of the AWG to be susceptible to shock and vibration, especially if the shock or vibration occurs in the PLC plane.
Delisle et al. in U.S. Pat. Nos. 6,701,043 and 6,798,048 disclose an AWG having a reflective input that permits variable coupling to compensate for AWG temperature drift. Referring to FIG. 1, an athermal reflective coupling 60 of a Delisle AWG 22 includes a thermally actuated pivot mechanism for supporting a mirror 32. The athermal coupling 60 includes a first arm 62 of a material having a first coefficient of thermal expansion and a second arm 64 of a different material having a second coefficient of thermal expansion. Each arm 62, 64 abuts a substrate edge 19. The first arm 62 supports a mirror frame 66, which is coupled to the second arm 64 at one side, and which carries a mirror 32 on another side of the first arm 62. A flex or pivot point 68 at the first arm 62 forms a rotation center, about which the mirror frame 66 pivots as shown by an arrow 21 in response to a differential thermal expansion of the first and second arms 62, 64. Light emitted by an input optical fiber 10 mounted to a holder 26 is collimated by a lens 30 and impinges on the mirror 32 as a collimated beam. The collimated beam is reflected back into the lens 30 at an angle determined by the pivot of the athermal coupling 60. The angle is translated by the lens 30 as an offset, thus shifting the input point at the input plane 20 of an input slab 12, and thereby at least partially compensating the thermal drift of the AWG 22.
The Samiec and Delisle AWG devices have drawbacks of vibration sensitivity and a relatively slow response to an abrupt temperature change. When the temperature changes quickly, thermal gradients between the PLC and the thermally expanding beams can cause time-varying wavelength drift. Furthermore, each PLC chip possesses slightly different thermal wavelength drift characteristics, requiring individual mechanical tuning of thermal response of each device, e.g. by adjusting individual lengths of the arms 62, 64. This makes the Samiec and Delisle AWG devices more difficult to mass produce.